1. Field of the Invention
The present invention relates to an imaging screen and readout system for mammography and, more particularly, to a mammography imaging system employing an electron trapping phosphor.
2. Description of the Related Art
Diagnostic mammography is a powerful tool for early detection of precancerous features in breasts. Historically, breast screening using X-rays was performed using radiographic film, without an intensifying screen. Although the method provided very high spatial resolution of breast features, a very high radiation dose to the patient was required to expose the film. Since the active layer on radiographic film is relatively thin (e.g., 20 micrometers), a high radiation exposure time was needed to allow the film to absorb a sufficient number of X-ray photons to obtain a high quality image. Presently, however, high resolution intensifying fluorescent screens have been developed which significantly reduce the radiation exposure to the breast With this approach, radiographic film is placed on top of the intensifying screen, which is farthest from the X-ray source. The X-rays which pass through the patient are first partially absorbed by the film, and the rest are more strongly absorbed by the intensifying screen. The X-rays absorbed by the intensifying screen cause the screen to fluoresce and further expose the film. In this way, the exposure to the patient is significantly reduced. In fact, the radiation exposure needed for the intensifying screen/film combination is very close to the minimum exposure needed to obtain a high quality image. That is, since there is inherently a statistical variation in the number of X-ray quanta, a specific minimum number need to pass through the patient and be absorbed by the imaging media to obtain an image with minimal noise. Therefore, increased sensitivity in the imaging media does not necessarily translate into a dose reduction, as this could produce a poor image where important features will become undetectable.
Therefore, the exposure sensitivity is not so much an issue, to the extent that radiation exposure is not increased relative to existing practices. Instead, it is important that the imaging media exhibit high contrast sensitivity and high spatial resolution to allow for the detection of small pre-cancerous features in breasts. These are usually microcalcifications, changes in tissue mass, or changes in blood flow, all low contrast features which must be detected. Unfortunately, since screen/film systems respond to linear contrast changes logarithmically, the previously stated features are not always easily detected. The logarithmic behavior of film basically means that a large contrast in the X-ray pattern will produce only a small contrast on the film. Therefore, a linear imaging media would be more desirable. Another important feature in the imaging media would be the ability to capture and record the X-ray image digitally. Being able to acquire images digitally would greatly simplify the archiving needs of the X-ray images, since they could be stored electronically. This would eliminate the need for massive physical storage of X-ray film. In addition, digital images can be manipulated to enhance features not easily detected with analog film images.